Conventionally, a load sensor formed with a strain sensitive resistor on a metallic board via a glass layer is proposed in Japanese Laid-open Patent H5-45238, etc.
FIG. 17 is a sectional view describing the structure of a conventional load sensor, which is an example of load sensor with a single layer wiring. In FIG. 17, glass enamel 2 is formed on elastic metal body 1, on which wiring 3a and strain sensitive resistor 4 are formed. However, when crystalline glass is used for glass enamel 2 in particular, it is well-known that increasing the degree of crystallization of the deposited crystal causes the surface to roughen.
In Japanese Laid-open Patent H6-137805, in order to reduce such surface roughening of crystallized glass, a metal alkoxide layer is formed on a crystallized glass layer to lessen the surface roughness. And it is proposed that a wiring pattern and strain sensitive resistor are formed thereon.
However, since a metal alkoxide layer is lower in strength and layer quality as compared with ordinary glass, it is difficult to use when high reliability and strength are required. Also, since a metal alkoxide layer itself and a strain sensitive resistor based on ruthenium oxide (RuO2) are liable to diffuse to each other, there is a problem such that the gauge factor (variation in resistance value against strain) of the strain sensitive resistor or TCR (temperature change rate of resistance value) are liable to be affected.
Here, the problems of crystallized glass proposed for a load sensor are further described in detail. Table 1 shows the composition of crystallized glass for single wiring layer proposed in Japanese Laid-open Patent H6-137805 and Japanese Patent No. 2979757. MgO is 16 to 50 wt (weight) %, BaO is 0 to 50 wt %, CaO is 0 to 20 wt %, La2O3 is 0 to 40 wt %, B2O3 is 5 to 34 wt %, SiO2 is 7 to 30 wt %, MO2 (M is at least one of Zr, Ti, Sn) is 0 to 5 wt %, and P2O5 is 0 to 5 wt %.
TABLE 1Glass componentTotal composition (wt %)MgO16 to 50BaO 0 to 50CaO 0 to 20La2O3 0 to 40B2O3 5 to 34SiO2 7 to 30M(Zr, Ti, Sn)O20 to 5P2O50 to 5
Also, besides these, the compositions of similar crystallized glass are proposed in Japanese Laid-open Patent H8-178768, Japanese Laid-open Patent H8-304200, Japanese Laid-open Patent H8-145814, and Japanese Laid-open Patent H9-8325.
However, when a strain sensitive resistor is directly formed on crystallized glass, there arises a problem that the characteristics are liable to vary because of surface roughening. In order to reduce the surface roughening of crystallized glass, for example, proposed in Japanese Laid-open Patent H6-13780 is to form a metal alkoxide layer on a crystallized glass layer for lessening the surface roughness. And to form a wiring pattern and strain sensitive resistor thereon is proposed.
However, a metal alkoxide layer is lower in strength and layer quality as compared with ordinary glass, and it is difficult to use when high reliability and strength are required. Also, since metal alkoxide layer itself and strain sensitive resistor based on RuO2 are liable to diffuse to each other, there arises a problem such that the gauge factor (variation in resistance value against strain) of strain sensitive resistor and TCR (temperature change rate of resistance value) are liable to be affected. Also, such materials are originally not intended to achieve the purpose of forming multi-layers, and unexpected problems are liable to arise when multiple layers are formed.
Also, in U.S. Pat. No. 5,898,359, forming an interface layer is proposed as a method of suppressing the influence of mutual diffusion between glass layer and resistor.
FIG. 18 describes the method of forming an interface layer, in which formed on elastic metal body 1 are glass layer 5, wiring 3b and strain sensitive resistor 4 via interface layer 6.
In Japanese Patent No. 3010166, proposed as the interface layer 6 are those containing granular alumina and granular zinc oxide which are suspended in glass matrix.
Table 2 is the composition of interface layer 8 proposed in Japanese Patent No. 3010166. Alumina is 15 wt (weight) % to 35 wt %, zinc oxide is 3 wt % to 6 wt %, glass frit mixture is 34 wt % to 53 wt %.
TABLE 2Glass componentTotal composition (wt %)Alumina15 to 35Zinc oxide (ZnO)3 to 6Glass frit mixture34 to 53
Table 3 shows the composition of the glass frit mixture. Lead oxide ranges from 50 wt (weight) % to 74 wt % (as interface layer, from 17 wt % to 39 wt %), boron oxide ranges from 10 wt % to 25 wt % (ditto, from 3 wt % to 13 wt %), and silica ranges from 8 wt % to 26 wt % (ditto, from 2 wt % to 14 wt %).
TABLE 3Glass componentTotal composition (wt %)Lead oxide (PbO)50 to 74Boron oxide (B2O3)10 to 25Silica (SiO2) 8 to 26Alumina (Al2O3) 0 to 12Titania, etc.0 to 3
Also, Table 4 shows the result of rearrangement of Table 2 and Table 3. The composition of interface layer 8 shown in Japanese Patent No. 3010166 can be re-calculated as follows: alumina is 15 to 41.4 wt %, zinc oxide is 3 to 6 wt %, lead oxide is 17 to 39.2 wt %, boron oxide is 2.7 to 13.8 wt %, silica is 0 to 6.4 wt %, and tetania, etc. is 0 to 1.6 wt %. Also, in Japanese Patent No. 3010166, proposed is a method of increasing gauge rates by including both of alumina and zinc oxide in glass.
TABLE 4Glass componentTotal composition (wt %)Alumina (Al2O3)  15 to 41.4Zinc oxide (ZnO)3 to 6Lead oxide (PbO)  17 to 39.2Boron oxide (B2O3) 2.7 to 13.8Silica (SiO2)  0 to 6.4Titania, etc.  0 to 1.6
However, in Japanese Patent No. 3010166, it is possible to stabilize the characteristics of the resistor, but there arises a problem as described later in FIG. 20 when wiring is formed in multiple layers because problems in forming multi-layer wiring are not taken into consideration.
For the purpose of matching with such strain sensitive resistor, proposed is non-crystalline glass. In Japanese Laid-open Patent H9-243472, proposed is to use borosilicate lead glass instead of crystallized glass for the manufacture of a load sensor with a single wiring layer. In Japanese Laid-open Patent H9-243472, proposed is to form a multi-layer insulating layer of three layers by repeating printing, burning, printing, and burning of an insulating layer of borosilicate lead glass having layer thickness of 30 μm or less per layer. However, in the case of borosilicate lead glass, as mentioned in Japanese Laid-open Patent H9-243472, it is well-known that burning a layer having thickness of about 40 μm is liable to cause internal cracking to take place.
Also, proposed in Japanese Laid-open Patent H11-326090 is a load sensor with single wiring layer. However, in the structure of a conventional load sensor with such a single wiring layer, since there is no freedom of designing the wiring, it is difficult to mount semiconductors and chip parts in high density on a load sensor board.
Further, as proposed in Japanese Laid-open Patent H11-351952 (U.S. Pat. No. 6,345,543), etc., a load sensor with various parts mounted on the board thereof is desirable to be mounted in a car. However, since an elastic metal body is directly fixed on the chassis of the car, it may be liable to pick up noise from the car. In order to solve such problem, for example, in Japanese Laid-open Patent 2003-97997, proposed is to form wiring in multiple layers for the purpose of improving EMI (electromagnetic interference) of a load sensor to be mounted in a car.
FIG. 19 is a sectional view showing an example of a load sensor with multi-layered wiring. In FIG. 19, a plurality of lower glass 7a, 7b and 7c are formed on elastic metal body 1, on which internal electrode 9a is formed, and further, a plurality of layers of upper glass 8a, 8b and 8c are formed thereon so as to cover the internal electrode 9a. And wiring 10a and strain sensitive resistor 12 are formed on the upper glass 8c, which are protected by protective layers 11a, 11b and 11c. To form such wiring of a load sensor in multiple layers is absolutely needed for miniaturization of the load sensor and for realizing higher function (for example, mounting semiconductor chips and various chip parts in high density on a load sensor board) besides the measure against EMI, and a conventional load sensor with single wiring layer is not enough to meet such requirement. When wiring is formed in multiple layers, as proposed in Japanese Laid-open Patent 2003-97997, it is possible to repeat printing and burning for each layer in order to form multiple layers, but there is a problem of manufacturing costs. Accordingly, desired is simultaneous burning of multiple layers.
FIG. 20 is a diagram for explaining the problems when a load sensor is multi-layered. The same portions as in FIG. 19 are given same reference numerals, and reference numeral 11d is a protective layer. Above all, explained is how crack 13 is generated between layers or in a layer. In the experiment executed by the inventor et al, it has been found that such crack 13 is liable to be generated when different kinds of glass are multi-layered.
For example, when glass layers being different in thermal expansion coefficient or non-crystalline glass and crystalline glass are combined in a sandwich fashion, cracks are liable to be generated between these layers. Also, it has been found that such problem is liable to arise when a plurality of glass layers are simultaneously burnt. Thus, when layers are individually burnt layer by layer, then problems hardly arise but the manufacturing cost becomes very high.
On the other hand, even if intended to realize cost down by burning many layers at one time, there will arise various problems such as ink leveling in multi-layer printing and matching of thermal expansion in multi-layer burning, and it has been unable to overcome such problems by employing glass materials and manufacturing processes for a load sensor conventionally proposed.
In prior art, since the load sensor proposed uses single-layer wiring, it is unable to cope with the market requirement for higher performance. Therefore, there has been a demand for a load sensor using multi-layered wiring. However, in the case of conventional materials, it is difficult to form multiple layers of wiring, and further, as the number of layers increases, there arises a problem of manufacturing cost down.